1. Field of the Invention
The present invention relates generally to strain gage type measuring devices. More particularly, the invention concerns a novel strain gage apparatus that includes a unique mounting mechanism which enables the apparatus to be directly mounted on a load-bearing structure without having to modify the structure thereby making the apparatus particularly useful for measuring the weight of articles and materials disposed within various types of structures such as tanks and hoppers.
2. Discussion of the Invention
Industries that use tanks and hoppers for batching inventory or mixing ingredients require a simple and inexpensive system for measuring the weight of articles or materials contained therewithin. In the past, such measurements have typically been accomplished through the use of load cells that are positioned beneath the legs of the tank or hopper. However, in order to install the load cells, the tank or hopper generally must be raised and, in certain instances, the legs of the tank or hopper must be cut or otherwise structurally modified in order to install the load cells. For example, in some instances, both cutting and welding operations are involved in the installation of the load cell. The use of conventional load cells for this purpose has several disadvantages, including the rather substantial cost, inconvenience, and time delays in the installation of the load cells.
Another prior art approach sometimes used for measuring the weight of articles or materials contained within supporting structures involves the use of strain gage devices that are mounted within drilled holes formed in the structural members of the supporting structure. In such instance, the strain gages are adapted to measure the deformation of the hole in the structural member as the load is increased. Typically the installation of such strain gage devices is difficult and reliable measurement of the deformation of the drilled hole is frequently suspect. Further, the use of such an approach is even more suspect when relatively small structures are involved.
Exemplary of yet another approach to making such measurements is the extensometer disclosed in U.S. Pat. No. 4,522,066 issued to Kistler et al. This device uses machined mechanical links to measure stresses in two axes. Temperature compensation is achieved mechanically by sensing both axes. With this arrangement, as expansion due to thermal changes occurs, the two axes nullify the effect of temperature. The effectiveness of such a system, however, requires that the mechanical links be very accurately machined. This precise machining can be both time consuming and costly. Further, the extensometer must be positioned accurately, both for correct load measurement and for cancellation of isotropic (thermally induced) strains.
U.S. Pat. No. 5,289,722 issued to Hugh S. Walker and Robert J. Dorgan addresses the problem of aligning and bonding strain gages to various types of structures without damaging either the structure or the strain gage. The strain gage assembly disclosed in this patent comprises a substrate having an adhesive applied to the majority of the undersurface thereof and terminating in an adhesive end line. A strain gage unit is adhered to the adhesive-bearing surface of the substrate. A selectively removable protective film is applied over at least a portion of one surface of the substrate. This requires a skilled technician familiar with strain gages and the specialized bonding processes involved.
As will be more fully appreciated from the discussion which follows, use of the novel apparatus of the present invention does not require any drilling or mechanical alteration to the structure being measured. Rather, the apparatus of the invention includes a simple and highly effective installation mechanism that enables a positive nondestructive interconnection of the apparatus with the structure being measured.
In one form of the invention, the apparatus includes a plurality of fully active, and prewired wheatstone bridges which effectively cancel temperature errors. Another important feature of the apparatus of the invention resides in the fact that it is specifically designed to be used in conjunction with conventional weighing instrumentation such as a Weigh Meter. More particularly, the apparatus of the invention has the ability to convert the analog signal to a digital signal for processing and correction and then to reconfigure the data back to analog signals for input to the Weigh Meter. In the preferred form of the invention, a summer-adder means functions to receive and appropriately sum the output of each of the wheatstone bridges.